Aromatic polycarbonates are well known thermoplastic resins. As like many other resins, polycarbonates require certain stabilizers so as to protect them from various phenomenon to which they are exposed, for example sun light, heat during processing, water vapor and the like. With respect to thermal stabilization properties, polycarbonates are known to yellow, a phenomenon related to the decomposition of the polymer chains during exposure to heat during processing. Because of its high Tg, glass transition temperature, polycarbonates must be processed at a high temperature to make them sufficiently plastic to obtain the desired shapes utilized in various applications. Typical stabilizers used over the years are phosphites and phosphonites.
Typical of the phosphites are the phosphites disclosed in Fritz U.S. Pat. No. 3,305,520. However these phosphites have been expanded to include other phosphites of far more complex structure. In fact these phosphites have been specifically included with other types of additives in polycarbonates to achieve specific desired effects. U.S. Pat. No. 4,221,728, Jaquiss, et al is one of these references. In this reference a polycarbonate is combined with a minor amount of an end blocked polyorgano hydrogen siloxane which confers antioxidant-reducing agent-thermostabilizing properties which are allegedly due to the presence of silicon bonded active hydrogen, see U.S. Pat. No. 4,221,728, incorporated by reference.
It has now been found that a non-phosphite or phosphonite phosphorus containing compound provides extremely fine thermal stabilization of polycarbonates, particularly those with low intrinsic viscosity as measured by Yellowness Index, Y.I. Very good hydrolytic stability as measured in the conventional testing systems is also expected. The stabilizer in the invention obtains these excellent results while substantially maintaining significant properties of the aromatic polycarbonate.